2. Why is the demand for diesel cars falling?

Prof David Bailey has said that fewer diesel cars are being driven off the forecourts because of "environmental pressures and consumer confusion".

The SMMT also says that consumer confidence has been hit. In the UK, as politicians have become more focused on air quality, potential customers have become more worried about restrictions on diesel cars.

The Volkswagen scandal showed that many diesel cars were producing higher levels of nitrogen oxides, NOx, on the road, than in laboratory tests.

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NOx is a significant contributor to pollution in cities and can cause respiratory diseases, making diesel cars a target of clean air campaigners such as Greenpeace.

Higher taxes on diesel vehicles might have hit demand too.

In the November Budget, the government announced a new levy on new diesel vehicles that failed to meet the latest emissions standards. The first-year road tax increase for diesel cars came into effect at the start of this month and will apply until around 2021.